This is a proposal to investigate proteins that contribute to the efficiency and accuracy of DNA double-strand break (DSB) repair. Ionizing radiation deposits energy along discrete tracks, resulting in clustered DNA damage and DSBs. The ability to repair these signature lesions is a major determinant of radiation sensitivity and resistance in both normal tissue and tumors. Manipulation of DNA repair pathways therefore affords a promising approach for improving the efficacy of radiotherapy. Recent work provides evidence for the novel involvement of a small family of human RNA binding proteins in DSB repair. These proteins are core components of paraspeckles, which are nuclear structures that are organized around a long noncoding RNA scaffold and that regulate gene expression by retaining adenosine-to- inosine hyper-edited mRNAs. Separate experiments indicate, however, that these proteins participate in both homologous recombination and nonhomologous end joining, which are the two main pathways of DSB repair in human cells. The three members of the family in humans-PSF, p54nrb, and PSPC1-rapidly relocalize to sites of induced DNA damage, suggesting the existence of a molecular switch that controls RNA versus DNA interaction. The hypothesis to be tested is that PSF and its partners are mediators of gene regulation and DNA repair that switch rapidly between RNA and DNA interaction modes following the induction of DNA damage. A unifying theme may be reliance on an intrinsic ability of PSF and its partners to promote pairing of distant nucleic acid segments. The first specific aim will be to test a prediction that a PSF7p54nrb complex promotes juxtaposition of opposing DNA ends in a loop structure. The second will be to examine repair functions of PSF and its partners more broadly using genetic manipulation of human cells. The third will focus directly on how PSF and its partners switch between RNA biogenesis and DSB repair modes and will investigate the therapeutic applicability of this mechanism. The proposed research is innovative, because the primary sequence and domain structure of PSF, p54nrb, and PSPC1 are unlike any previously characterized DSB repair proteins. The work is scientifically significant, because it explores a previously unsuspected link between DSB repair and the biology of non-coding RNAs, which is an interesting and topical area in cell biology. Finally, the work has potential clinical and translational impact, because of the possibility that therapeutic RNAs might be developed to influence switching between RNA biogenesis and DNA repair modes to alter clinical radiation response.